It is known that acrylate esters which are useful as polymerizable monomers can be produced by reacting about 2 moles of acrylic acid or methacrylic acid with 1 mole of an organic diol. U.S. Pat. No. 3,645,984 discloses acrylate esters which are produced by reacting about 2 moles of acrylic acid with 1 mole of an organic diol of the formula: ##STR1## wherein R.sub.3, R.sub.4, R.sub.9, and R.sub.10 are selected from the group consisting of H, alkyl, aryl, cycloalkyl, substituted alkyl, substituted aryl, and substituted cycloalkyl groups, R.sub.5, R.sub.6, R.sub.7, and R.sub.8 are selected from the group consisting of H, alkyl, aryl, and cycloalkyl, and k, l, m, n, q, and p are whole numbers having values from 0 to 5. One such acrylate ester which has been used as a crosslinking agent in radiation-curable coating compositions is 3'-acryloxy-2',2'-dimethylpropyl 3-acryloxy-2,2,-dimethylpropionate. Incorporation of this acrylate ester into radiation curable coating compositions allows the skilled worker in the art to produce cured coatings having a hard, mar-resistant finish. However, certain problems have been encountered in the known processes for producing acrylate esters of the diols of formula I, particularly in the production of low color acrylate esters for use in clear coatings. It has been found that the acrylate esters of the diols of formula I and acrylic acid tend to undergo polymerization under the same conditions which promote the esterification reaction between acrylic acid and the diol. Therefore, it is essential that precautions be taken to ensure against polymerization of the acrylate ester product and acrylic acid in the reaction vessel during esterification of the diol with acrylic acid. While methylene blue and phenothiazine have been found to be effective polymerization inhibiting agents in the esterification reaction, they are highly colored materials and the resultant product is unsuitable for use in clear coatings. The color cannot be removed by distillation of the product because the acrylate ester product generally must be recovered from the reaction mixture as a residue rather than as a distillate due to its strong tendency to polymerize at its boiling point. Even under a 5 mm. Hg vacuum, for example, 3'-acryloxy-2',2'-dimethylpropyl 3-acryloxy-2,2-dimethylpropionate polymerizes in about 5 minutes at its boiling point. For this reason these compounds are generally recovered at residue acrylate esters. Neither is the standard technique of color adsorption on clay useful, since this technique is usually limited to removal of color causing agents from non-polar media and the acrylate ester is a relatively highly polar medium. Moreover, the viscosity of the acrylate ester is such that it is difficult to pass through clay.
The conventional phenolic type polymerization inhibitors, such as hydroquinone, the monoalkyl ethers of hydroquinone and the alkylated hydroxyanisoles are ineffective or impractical in the production of low color residue acrylate esters of the diols of formula I by the processes of the prior art. The prior art generally discloses preferred reaction temperatures for the esterification reaction of 95.degree. C. and higher (see, e.g., U.S. Pat. No. 3,645,984). At these temperatures the monoalkyl ethers of hydroquinone and the alkylated hydroxyanisoles are essentially ineffective in inhibiting polymerization. Hydroquinone, at concentrations on the order of 10,000 ppm usually, though not always, inhibits polymerization in the production of 3'-acryloxy-2',2'-dimethylpropyl 3-acryloxy-2,2-dimethyl propionate at reaction temperatures exceeding 95.degree. C., but the product is highly colored. The color can be removed by a caustic wash at a pH of about 12, but this requires additional process steps and creates a problem in that the products of reaction frequently emulsify under strongly alkaline conditions, making recovery of the residue acrylate ester product difficult.